|Publication number||US20050018732 A1|
|Application number||US 10/741,494|
|Publication date||Jan 27, 2005|
|Filing date||Dec 19, 2003|
|Priority date||Dec 19, 2002|
|Publication number||10741494, 741494, US 2005/0018732 A1, US 2005/018732 A1, US 20050018732 A1, US 20050018732A1, US 2005018732 A1, US 2005018732A1, US-A1-20050018732, US-A1-2005018732, US2005/0018732A1, US2005/018732A1, US20050018732 A1, US20050018732A1, US2005018732 A1, US2005018732A1|
|Inventors||Aaron Bond, Newton Frateschi, Jiaming Zhang|
|Original Assignee||Aaron Bond, Frateschi Newton C., Jiaming Zhang|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Referenced by (20), Classifications (15), Legal Events (4)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This application is related to and claims the benefit of U.S. Provisional Application No. 60/434,629 entitled UNCOOLED AND HIGH TEMPERATURE LONG REACH TRANSMITTERS, AND HIGH POWER SHORT REACH TRANSMITTERS filed on Dec. 19, 2002.
The present invention concerns a design for producing uncooled, high-powered transmitters and transponders for optical communications systems. This design may also allow the use of reduce form factor packages.
Optical transmitters and transponders are used extensively in many communication systems, which may extend over large distances. It is desirable to be able to transmit optical signals over these large distances. Signal loss within optical fibers limits the distance that an optical signal of a certain power level may be transmitted effectively. Scattering and absorption of the light may be a major source of signal loss in optical fibers. In-line amplifiers to boost the optical signal may increase the distance the signal may be transmitted, but these amplifiers may amplify noise as well as the signal, reducing their efficiency. Dispersion is also a source of signal degradation in an optical fiber. Transponders, which receive an optical signal from an input fiber and then retransmit the signal on an output fiber, are another device that may increase the distance a signal may be transmitted in an optical communications network. Transponders include both a receiver and a transmitter and are, therefore, a relatively complicated and expensive component. Also, the process of converting the optical signal to an electrical signal, then back to an optical signal, may introduce errors in the signal. Additionally, both in-line amplifiers and transponders require power sources and introduce coupling losses, which lessen their effectiveness.
It is desirable to design transmitters (and transponders) with a long reach (the distance the optical signal may be transmitted without excess degradation to the signal quality). Different sub-components may be used to create transmitters and transponders with different fiber reaches. The output power of the laser source and/or the sensitivity of the receiver may be increased to increase the reach of a transmitter or transponder. The choice of wavelength band for the communication system also plays a role in determining the sub-components to be used in the system. Optical fiber has different loss for different wavelengths, as well as link lengths. For light having a wavelength of 1.3 μm the loss of power in an optical fiber is typically estimated to be 0.5 dB/km. For light at 1.55 μm the corresponding optical loss is typically estimated to be 0.25 dB/km. This may not be the only factor in selecting a wavelength in an optical communications system, though. The choice of wavelength for a particular fiber reach is determined by a total link budget. This link budget is defined by the output power of the transmitter, the total loss through the fiber and connectors, dispersion power penalty, and the receiver sensitivity.
Choices of receivers include; a PIN photodiode receiver (having a sensitivity of −16 dBm), a standard avalanche photodiode (APD) receiver (sensitivity=−21 dBm), and a high end APD receiver (sensitivity=−26 dBm). Although the APD's provide superior sensitivity, they are also significantly more expensive.
In an exemplary optical communication system, a 1.3 μm directly modulated laser (DML) with −4 dBm launch power may be paired up with a PIN photodiode receiver to cover distances up to 12 km. For longer reaches, higher power laser sources, and/or more expensive APD's may be required. Higher power laser sources typically require cooling systems to maintain their performances. Also, higher power laser sources are impractical to operate as DML's. Therefore, external modulators, such as electroabsorption modulators (EAM's) and LiNbO3 Mach-Zehnder modulators (MZM's), are typically used for higher powered laser sources, but the wavelength sensitivity of these modulators may raise additional issues. Table 1 illustrates transmitter sub-components typically used in existing 10 Gb/s applications.
TABLE 1 Reach Laser Source Modulator <600 m 1.3 μm-uncooled Fabry-Perot laser None (i.e. DML) <12 km 1.3 μm-uncooled distributed None feedback laser 20-40 km 1.55 μm cooled laser Integrated EAM 40-80 km 1.55 μm cooled laser Integrated EAM 1.55 μm cooled laser MZM 1.55 μm cooled laser Amplified EAM
ITU SONET specification standards for 1.3 μm optical communications systems have been set to assist in designing these systems as shown in Table 2.
TABLE 2 Received Dispersion Standard Output Power Reach Power penalty OC192, SR-1 −4 dBm 12 km −12 dBm 1 dB OC192, IR-1 −1 dBm 24 km −13 dBm 1 dB OC192, LR-1 10 dBm 48 km −13 dBm 1 dB OC192, VR-1 10 dBm 72 km −24 dBm 1 dB
Presently sub-components for short and intermediate reach applications (SR-1 and IR-1) are available. A 1.3 μm DML with −4 dBm launch power may be paired up with a PIN photodiode receiver to meet the SR-1 standard. An uncooled 1.3 μm laser integrated with an EAM (EML) or a high performance uncooled 1.3 μm DML paired with a PIN photodiode receiver may meet the IR-1 standard. No viable single transmitter solution is yet available to meet the LR-1 and VR-1 standards. The LR-1 standard may be met with an external SOA and a PIN photodiode receiver and the VR-1 standard may be met with a cooled external SOA and an APD receiver.
In addition to the 1.3 μm specifications, ITU also defines specifications for 1.55 μm systems as shown in Table 3. 1.55 μm operation has less loss in the fiber; however, dispersion in the optical fiber is a greater issue than at 1.3 μm.
TABLE 3 Received Dispersion Standard Output Power Reach Power penalty OC192, SR-2 −4 dBm 20 km −12 dBm 2 dB OC192, IR-2 −1 dBm 40 km −14 dBm 2 dB OC192, IR-3 −1 dBm 40 km −13 dBm 1 dB OC192, LR-2a −2 dBm 80 km −26 dBm 2 dB OC192, LR-2b 10 dBm 80 km −14 dBm 2 dB OC192, LR-3 10 dBm 80 km −13 dBm 1 dB OC192, VR-2a 10 dBm 120 km −25 dBm 2 dB
A 1.55 μm DML with −4 dBm launch power may be paired up with a PIN photodiode receiver to meet the SR-2 standard. A cooled 1.55 μm EML paired with a PIN photodiode receiver may meet the IR-2 and IR-3 standards. The LR-2a standard may be met with cooled 1.55 μm EML, or a laser integrated module configuration (a cooled 1.55 μm laser coupled to an amplified EAM, e.g., a T-Networks LIM™ package), paired with a high performance APD receiver. The LR-2b and LR-3 standards may be met with: a cooled 1.55 μm EML, a laser integrated module, or a cooled 1.55 μm laser coupled to an MZM; an external SOA or erbium doped fiber amplifier (EDFA); and a PIN photodiode receiver. The VR-2a standard may be met with: a cooled 1.55 μm laser coupled to an MZM; an external SOA or EDFA; and a PIN photodiode receiver.
In the short distance market cost, size and power dissipation are important considerations. 10 Gbit Ethernet has similar issues in terms of reach, form factor, and power dissipation tradeoffs. External modulator solutions are not desirable for these markets to get high power. Presently, DML's cannot produce high enough power in uncooled operation at a reasonable reliability to be practical solutions for LR-1 and VR-1 requirements. Cooled solutions are also not desirable in this market owing to the additional power and heat dissipation requirements of these systems. Although DML's may be used at 1.3 μm, they are not used extensively for 10 Gb/s signals at 1.55 μm. DML's have inherently high chirp compared to externally modulated lasers and are, therefore, not suited well for long distance transmission at 1.55 μm. This is because, while the dispersion of the optical fiber is negligible at 1.3 μm, it is relatively high (typically about 17 ps/nm) at 1.55 μm.
Several different form factor standards of transponders and transceivers have been created including: 300 pin MSA (3.5×5″); 300 pin SFF (2×3″); XenPak (36×120 mm); X2/XPAK (76×36 mm); and XFI/XFP (Small, <18 mm wide). Generally, smaller form factor packages are desirable to allow miniaturization of the system, but a smaller factor package may have difficulty dissipating heat generated within it. Each of these standard package form factors is rated to be able to dissipate a certain amount of heat during operation: MSA, 15 W; SFF, 9 W; XenPak, 9 W; X2/XPAK, 4 W; and XFI/XFP; 2-3.5 W.
In a typical cooled laser solution, the laser may have a minimum operating temperature of 25° C., and a desired maximum case temperature of 75° C. Such a cooled laser may need to dissipate close to 2 W of heat. The laser is not the only source of heat within the package that must be dissipated by the package. Electronics within the package and external modulators for long reach transmitters may also generate significant heat. Transponders include additional components that may generate heat. Thus, the XenPak form factor is the smallest desirable form factor package that may be reasonably used in this example. It is, therefore, undesirable to use a cooled laser in any of the smaller form factor solutions.
As described previously, at present there are no uncooled or low power dissipation solutions for reaches greater then 20 km at 1.55 μm and 10 Gb/s. The shorter reach systems typically operate at 1.3 μm. For intermediate reach (40 km) and long reach (80 km) applications, uncooled 1.55 μm EML's may be desirable, but are not available in the market today.
An exemplary embodiment of the present invention is a method for substantially maintaining, within a predetermined temperature range, the performance, i.e., output power, extinction ratio, and dispersion penalty, within system limits of an uncooled optical transmitter that includes a laser and an electroabsorption modulator (EAM). α represents the small signal chirp of the device. It is measure at different modulator biases being a measure of the amount of frequency modulation induced by the amplitude modulation of the modulator. Therefore, α(Vbias) is a function that can be used to monitor the amount and the signal of the chirp imposed to the modulator by the modulation and the consequences caused to the dispersion penalty. Particularly, the α crossing point (the voltage at which the small signal a curve crosses through zero) may be used as a reference to maintain a constant dispersion penalty in the system. The small signal α crossing points at two temperatures within the predetermined temperature range (or, alternatively, at the lowest temperature in the temperature range) are determined. An EAM bias voltage versus temperature control function is calculated based on the two small signal α crossing points (or, alternatively, the one small signal α crossing point) and the bias voltage of the EAM is adjusted based on this control function, substantially maintaining the dispersion penalty of the transmitter within the predetermined temperature range.
Another exemplary embodiment of the present invention is an uncooled long reach optical transmitter, including an uncooled laser source, an uncooled semiconductor optical amplifier (SOA) optically coupled to the uncooled laser source, and an uncooled EAM optically coupled to the uncooled SOA. The uncooled laser source produces a laser beam, which is amplified by the uncooled SOA. The amplified laser beam is modulated by the uncooled EAM.
An additional exemplary embodiment of the present invention is a method for substantially maintaining, within a predetermined temperature range, the output power of an uncooled optical transmitter, which includes a laser and an SOA. An initial laser bias current and an initial SOA bias current are set. The output power of the uncooled optical transmitter is measured and the SOA bias current is adjusted based on the measure output power to substantially maintain the output power of the uncooled optical transmitter over the predetermined temperature range.
A further exemplary embodiment of the present invention is an uncooled long reach optical transponder, including a PIN photodiode receiver, modulation circuitry electrically coupled to the PIN photodiode receiver, an uncooled laser source, an uncooled SOA optically coupled to the uncooled laser source, and an uncooled EAM optically coupled to the SOA and electrically coupled to the modulation circuitry. The uncooled laser source produces a laser beam, which is amplified by the uncooled SOA. The modulation circuitry is adapted to provide a modulation signal responsive to an optical signal incident on the PIN photodiode receiver. The uncooled EAM modulates the amplified laser beam in response to the modulation signal to form an output optical signal of the uncooled long reach optical transponder.
Yet another exemplary embodiment of the present invention is a method for improving the reliability of an uncooled long reach optical transmitter operating substantially at a predetermined output power. The uncooled long reach optical transmitter in this exemplary method includes a laser and an SOA. The laser is operated at reduced bias current injections to produce a reduced power laser beam, thereby improving the laser reliability. The SOA bias current is controlled so that the SOA amplifies the reduced power laser beam to substantially maintain the predetermined output power. The SOA is sufficiently long to provide this amplification, while maintaining a reduced current density within the SOA, thereby improving the SOA reliability.
Still another exemplary embodiment of the present invention is a method for manufacturing a monolithic laser integrated module for use in an uncooled long reach optical transmitter. A substrate base having a substrate base index of refraction is provided. A grating layer is formed over the substrate base. The grating layer has a grating index of refraction, which is different from the substrate base index of refraction. The grating layer is defined and etched to form a grating base section having a grating period. A top substrate layer is formed over the substrate base and the grating base sections. The top substrate layer has a substrate index of refraction, which is different from the grating index of refraction. A quantum well layer is formed on the top surface of top substrate layer. The quantum well layer, which has a waveguide index of refraction different from the substrate index of refraction, includes a plurality of sub-layers forming a quantum well structure. Each of these sub-layers includes a waveguide material. A semiconductor layer is formed on the quantum well layer. The semiconductor layer has a semiconductor layer index of refraction different from the waveguide index of refraction. The quantum well layer and the semiconductor layer are defined and etched to form a distributed feedback laser section, an SOA section, and an EAM section in the quantum well layer. A distributed feedback laser electrode, an SOA electrode, and an EAM electrode are deposited on the semiconductor layer in positions corresponding to portions of the distributed feedback laser section, the SOA section, and the EAM section of the quantum well layer, respectively.
The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures:
One exemplary embodiment of the present invention is a transmitter, or transponder, capable of 80 km 10 Gb/s performance with 0 dBm-modulated power in an uncooled application, and able to be packaged in a small form factor package, such as SFF, XenPak, X2/XPAK, or XFI/XFP. This device may enable long reach, small form factor solutions for optical communication systems, providing small form factor transmitters and transponders for OC192 standard IR-2, IR-3, LR-2a, LR-2b, and LR-3 applications.
Another exemplary embodiment of the present invention is a design of a transmitter, or transponder, which is small, and operates at a wavelength of 1.3 μm. This exemplary design can enable transmitters and transponders for LR-1, and VR-1 links in the smaller form factor packages where power dissipation is a significant issue and direct modulated lasers cannot achieve the desired output power.
As described above, it is desirable to design a long reach transmitter, which may be operated without external cooling to allow smaller form factor packaging. In an exemplary embodiment of the invention, this goal may be achieved using a laser integrated module. Exemplary laser integrated module configuration 300, shown in
Desirable to proper operation of any communications systems is reliability. In an uncooled optical communications transmitter application, reliability is an increased concern. Performance of many electro-optical devices, such as lasers, degrades over time and this degradation is generally accelerated at higher temperatures. However, the reliability of semiconductor lasers may be significantly increased as the laser power is reduced, as well. Graph 100 in
If the overall reliability of the laser were to be improved through improved design or fabrication methods, then exemplary transmitters and transponders, which may be operated at a constant high temperature, may result. The thermoelectric cooler (TEC) would then only need to provide minimal cooling to such a laser source, possibly decreasing the TEC power demands to <0.5 W of heat dissipation. This may provide a viable solution for transmitters utilizing some of the larger form factor packages, but the TEC may still be too large for the ultra small form factor solutions. Therefore, for ultra small form factor packages, uncooled laser operation is desirable, even though uncooled operation leads to issues of stable operation over a temperature range and device control, as well as reliability.
The reliability parameters for a 10 Gb Ethernet system are preferably at least a median time to failure (MTTF) of 15 years at 50° C. The reliability requirement for SONET/SDH is typically 1-3% cumulative failure rate (CFR) over 10 years at a chip temperature of 50° C. However, as shown in
An uncooled long reach optical transmitter including a laser and a SOA is provided, step 200. An external modulator is also desirably included in the uncooled long reach optical transmitter, as well as, an optical isolator to desirably reduce feedback into the laser. FIGS. 3A-C illustrate possible exemplary configurations of these sub-components. Lenses 304 may also be included to improve optical coupling between the subcomponents of the uncooled long reach optical transmitter, and/or output fiber 312. Although FIGS. 3A-C show lasers 302 as distributed feedback (DFB) lasers and external modulators 310 as an EAM's, it is contemplated that other semiconductor lasers, such as Fabry-Perot and distributed Bragg reflector (DBR) lasers, and external modulators, such as Mach-Zehnder modulators (MZM), may alternatively be used in various embodiments of the present invention.
The laser is operated at a reduced output power level, step 202. As shown in
TABLE 4 Laser Coupling SOA EAM Coupling Power Power Loss Gain Loss Loss Output Cooled 13 dBm −4 dBm 4 dB −4 dB −2 dB 7 dBm Uncooled 3 dBm −6 dBm 13 dB −4 dB −3 dB 3 dBm
The coupling losses are estimated to increase in the uncooled example due to the large range of thermal expansion and larger peak wavelength variation anticipated during uncooled operation over a varying temperature environment. These temperature induced variations may lead to difficulties in optimizing optical coupling between the subcomponents of the exemplary transmitter. To reduce the temperature range for operation, high forced operating temperature of the laser may be possible to improve the coupling losses without adding significantly to the heat that must be dissipated by the package. A resistive heating element may even be used, saving space and cost as compared to a thermoelectric cooler, but the time constants associated with temperature settling using this means may be undesirably long for some applications.
It is also noted that at a constant bias current the output intensity of the laser may vary significantly. To overcome this problem, the SOA bias current may be controlled to variably amplify the reduced output power of the laser and maintain a substantially constant output power level, step 304. This allows the exemplary uncooled long reach optical transmitter to maintain a fixed output intensity across a wide temperature range, at least 80° C.
High temperatures also reduce the reliability of SOA's. Another important factor in the reliability of an SOA is the current density in its active layer. The higher the operational current density the SOA, the higher the cumulative failure rate of the SOA, similar to the effect of the output power on laser reliability. Therefore, it is desirable to operate an SOA, particularly one operated at elevated temperatures, with the lowest possible current density. In the present exemplary embodiment, the SOA is desirably sufficiently long to provide the desired amplification, while maintaining a reduced current density within the SOA, thereby improving the SOA reliability.
It is also noted that the dispersion penalty of the exemplary transmitter may be affected by the chirp generated in the EAM, which may also be sensitive to the operating temperature of the transmitter. This dispersion penalty may be substantially controlled by adjusting the EAM bias voltage, as discussed below with reference to
By improving the reliability of both the laser and SOA the present method may significantly improve the overall reliability of an exemplary uncooled long reach transmitter, or transponder. This improved reliability is an important step toward desirably designing a long reach optical transmitter capable of being mounted in a small form factor package.
An exemplary approach to an uncooled long reach transmitter design is the combination of an SOA and a laser to achieve high output power with increased reliability, the method of
Alternatively, DFB, or DBR, laser 300 and SOA 308 may be formed monolithically with separate optical isolator 306 and EAM 310, exemplary uncooled long reach transmitter 314 of
The SOA reliability is dependent primarily on current density within the SOA. By making the SOA longer, higher gain may be achieved while maintaining operation at a constant current density, and reliability. Thus, the DFB laser may be operated at a lower power and the SOA gain may be increased to compensate, thereby maintaining reasonably high output power for the transmitter, or transponder, without sacrificing reliability. In this way, increased reliability in an uncooled transmitter or transponder may be achieved by operating the laser and the SOA at low-current density.
Also of importance for uncooled applications is substantially stable performance over a temperature range, as uncooled systems may be more susceptible to changes in temperature during operation than cooled systems. The addition of the SOA to the exemplary transmitter enables an exemplary configuration to utilize independent control of the SOA gain by adjusting the SOA current. Increasing the drive current to the laser to maintain output power as temperature increases may undesirably affect the wavelength, linewidth, and noise of the laser output, as well as its reliability. Therefore, adjusting the SOA gain may provide a more desirable method to flatten the output power of the transmitter over a temperature range.
An optical power detector may be optically coupled to the uncooled EAM to monitor the average output power of the modulated laser beam. Evanescent coupling or a small optical fiber pick off may be used to minimize the power loss due to this power monitoring. Feedback from the optical power detector may be used to control the bias current of the SOA to maintain a substantially constant average output power level.
A temperature insensitive wavelength detector, such as that described in U.S. patent application Ser. No. 10/337,443, INTEGRATED, TEMPERATURE INSENSITIVE WAVELENGTH LOCKER FOR USE IN LASER PACKAGES, may also be optically coupled to the uncooled EAM to monitor output wavelength of the modulated laser beam. This may allow control of the laser bias current to reduce wavelength variation of the exemplary uncooled transmitter.
A resistive heating element may also be coupled to exemplary transmitters 300, 314, and 316 to allow limited temperature control at elevated temperatures. A temperature sensor may also be desirable in this embodiment. Alternatively, in the case of a transmitter which includes a temperature insensitive wavelength detector, the heater may be used, alone or in conjunction with the laser bias current, to reduce wavelength variation of the exemplary high temperature transmitter.
An uncooled long reach optical transponder may be formed by including a PIN photodiode receiver and modulation circuitry inside the package. The modulation circuitry is desirably adapted to provide a modulation signal which is responsive to an optical signal incident on the PIN photodiode receiver. When generating the modulation signal, the electrical signal from the PIN photodiode receiver may be filtered to remove noise and/or amplified by the modulation circuitry. This modulation signal is used to drive EAM 310 modulating the amplified laser beam to form the output optical signal of the uncooled long reach optical transponder.
Monolithic laser integrated module 318 in exemplary uncooled long reach transmitter 316 of
Monolithic laser integrated module 318 is desirably grown by low-pressure metal-organic chemical vapor deposition of III/V materials. To enable longer wavelength operation, laser section 302 and SOA 308 may be grown with an enhanced deposition rate by selective area growth (SAG). The epitaxial structure for monolithic laser integrated module 318 consists of a separated confinement (SCL) design with an active region employing quantum wells formed of layers of III/V materials, which may be compressively strained. Graded layers of III/V material are desirably employed between the quantum wells and cladding layers to minimize carrier accumulation and power saturation. The quantum wells and graded layers may desirably be formed of InGaAlAs and the cladding layers of InP. Exemplary cladding layer compositions and doping profiles were reported in LOW INSERTION LOSS AND LOW DISPERSION PENALTY InGaAsP QUANTUM WELL HIGH SPEED ELECTROABSORPTION MODULATOR FOR 40 GB/S VERY SHORT REACH, LONG REACH AND LONG-HAUL APPLICATIONS, by W. Choi, et al. in IEEE Journal of Lightwave Technology, 2002, vol. 20, pp. 2052-2056. After the waveguide formation, the sample may be planarized with polyimide (not shown) to reduce a metal pad capacitance and standard p and n contacts are deposited by electron beam deposition. Antireflection coatings may be desirably deposited on the output facet after cleaving.
The process begins with a planarized substrate base, step 500. Substrate base 400 is preferably formed of a III/V semiconductor, such as InP, GaAs, or InGaAsP. The substrate base may also be formed of multiple layers such as GaAs grown on silicon or alumina. A grating layer is formed over substrate base 400, step 502. Metal organic chemical vapor deposition (MOCVD) is one exemplary method that may be used for deposition of this grating layer, but other epitaxial deposition techniques may also be employed, such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and chemical beam epitaxy (CBE). The grating layer desirably has a sufficiently larger refractive index than substrate base 400 to provide the scattering necessary for the optical grating section of DFB, or DBR, laser section 302 of the exemplary laser integrated module. This grating layer is also desirably formed of a material of the same family as substrate base 400. For example, an InP grating layer may desirably be formed on an InGaAsP substrate base.
A grating portion of the grating layer is defined and etched to form grating base 600 with a series of parallel lines, step 504. These parallel lines may desirably be formed using a photolithographic technique, such as phase masking or e-beam writing, and a wet chemical etch. Alternatively, a dry etch technique, such as reactive ion etching, may be used. Grating base 600 is formed with a grating period selected to provide the desired feedback for laser section 302.
Top substrate layer 402 is formed over etched grating base 600 to form optical grating 404 and this layer is then planarized, step 506. MOCVD or another epitaxial deposition technique may be employed. It may be desirable for the same deposition technique to be used to form all of the semiconductor layers in this exemplary method. Top substrate layer 402 desirably has a sufficiently smaller refractive index than grating base 600, and preferably similar to substrate base 400, to provide the scattering necessary for optical grating 404 of exemplary monolithic laser integrated module 318.
Substrate base 400 and top substrate layer 402, shown in
An alternative exemplary method may be employed to form optical grating 404. In this alternative method, a grating portion of substrate base 400 is defined and etched to form a grating base with a series of parallel lines. The grating layer is formed over these etched grating bases to form optical grating 404, using MOCVD or another epitaxial deposition technique. This layer is then planarized. No top substrate layer is necessary. Substrate base 400 also functions as the low refractive index portion of optical grating 404, in this alternative embodiment.
Once optical grating 404 is formed, a plurality of sub-layers making up quantum well layer 406 are grown, step 508. MOCVD or another epitaxial deposition technique may be employed. The quantum wells and barriers may desirably be composed of InxAlyGa(1-x)As(1-y) materials, as well as InxGa(1-x)AsyP(1-y) and InxGa(1-x)As materials. Specific selections of x and y depend on the desired bandgap and strain, if any, desired. These sub-layers may also be formed by other permutations of alloys formed from III/V elements. The quantum wells and barriers of quantum well layer 406 desirably have a sufficiently larger refractive index than the top substrate layer 402 so that the quantum wells and barriers may act as a waveguide. In an exemplary embodiment the quantum well layer may desirably include strained InGaAlAs sub-layers and/or graded InGaAlAs sub-layers.
It is noted that one property of quantum well structures, which may be desirably exploited in this exemplary method, is that as the thickness of the quantum well increases the band gap or energy of the absorption peak decreases. Bias voltages applied to quantum well structures may also shift the band gap of the structure. By using selective area growth it is possible to grow a single multi-layer quantum well structure of varying thickness, and thus having a varying zero bias band gap energy. This may desirably allow tuning of the biased band gaps of the section of exemplary monolithic laser integrated module 318 to improve the efficiency of the monolithically integrated sub-components.
To include this alternative exemplary feature step 504 includes the formation of at least one patterned growth retarding mask on a laser area and an SOA area of the top surface of the top substrate layer. Materials which retard growth of III/V materials, such as SiN or SiO2, make up the growth retarding mask(s). The growth retarding mask may be formed and patterned using any standard techniques known in the semiconductor industry. The patterned growth retarding mask(s) may be formed as two rectangular regions with a channel between disposed along longitudinal axis of the monolithic laser integrated module in laser section 302 and SOA section 308. For an exemplary monolithic laser integrated module with a 2 μm wide waveguide, a 15 to 20 μm channel is desirable to provide substantial flatness of the layers in a transverse direction. Depending on the profile desired for the waveguide layer, other patterns, such as paired trapezoids or triangles, may be used. A larger number of regions may also be used.
When the plurality of sub-layers making up the waveguide layer are grown, the growth rate near the growth-retarding regions is enhanced owing to gas phase diffusion and surface diffusion of the reactants in the MOCVD reactor away from the growth-retarding regions. The quantum wells layers thus deposited are made thicker in laser section 302 and SOA section 308 than in EAM section 310 of the device owing to the growth-retarding masks.
Next semiconductor layer 408 is formed over waveguide layer 406, step 510. This step of the fabrication process is illustrated in
Note that if the thicknesses of the sub-layers of quantum well layer 406 are varied using selective area growth, then the thickness of semiconductor layer 402 may be varied as well, if the growth retarding masks are not removed before step 510.
Step 512 defines the waveguide and component structure of exemplary monolithic laser integrated module, for example, by selectively forming photoresist over the desired waveguide and component structure. This structure includes a mesa waveguide structure with a laser section, an SOA section, and an EAM section arranged longitudinally along the waveguide.
Next quantum well layer 406 and cladding layer 408 are etched to form this structure, step 514. Steps 512 and 514 may be performed using standard wet or dry etch techniques. Although steps 512 and 514 are shown following step 510 in
Once the waveguide and component structure is formed, p-type ohmic contacts are deposited on semiconductor layer 408 to form laser electrode 410, SOA electrode 412, and EAM electrode 414, step 516, as shown in
The device may be cleaved, step 518, to form the rear facet of the DFB, or DBR, laser and the output port of the exemplary monolithic laser integrated module 318. Steps 516 and 518 may be carried out by any of a number of standard semiconductor fabrication techniques known to those skilled in the art. The output port may be anti-reflection coated to reduce losses and reflections. Alternatively the output port may be formed using a low-loss optical coupling technique such as a buried facet. The cleaved rear facet of the laser functions as a reflector for the laser. The relatively high index of refraction of the waveguide materials desirably leads to approximately 30% reflectivity for this surface. This reflectivity may be increased by coating this surface with several dielectric layers to form a dielectric mirror and/or metallization layer, if desired.
As noted above, output power flatness of an exemplary uncooled long reach transmitter may be achieved by operating the integrated SOA as a variable amplifier. A control circuit, such as a micro-controller or a digital signal processor, may be used to monitor a temperature sensor, such as a thermistor mounted in the package, and determine the desired current to be applied to the SOA from a look-up table based on the temperature sensor reading. The SOA current may then be adjusted to maintain a constant optical output power. Thus, using this exemplary laser integrated module architecture in uncooled applications may allow for use of ultra-small form factor packaging. Alternatively, the output power may be directly monitored and the bias current of the SOA adjusted accordingly.
The output power of the uncooled optical transmitter is measured, step 804, desirably using evanescent coupling or a low loss optical pickoff. It is contemplated that the output wavelength of the uncooled optical transmitter may also be measured at this step. The laser bias current may be adjusted to maintain a substantially constant output wavelength for the uncooled optical transmitter. The SOA bias current may be dynamically adjusted based on the measured output power of the uncooled optical transmitter to maintain a substantially constant output power level, step 806.
By controlling the bias point of an exemplary uncooled optical transmitter, the dispersion penalty over temperature through 40 or 80 km of fiber may also be controlled. Controlling the dispersion penalty over temperature is desirable for achieving uncooled long reach operation and may desirably be accomplished in the exemplary laser integrated module architectures, such as those shown in
Although exemplary monolithic laser integrated module 318 of
The optical extinction curves of an EAM change as a function of temperature. Graph 900 of
The dispersion penalty of an EAM is based on the amount of chirp introduced during modulation. It is desirable for the EAM to be able to operate with a similar dispersion penalty over a temperature range. An algorithm may be used to change the voltage applied to the EAM as a function of temperature to maintain a similar dispersion penalty for the signal.
In exemplary graph 1000, α represents the small signal chirp of the device. This is a measure of the amount of frequency modulation induced in the output by small amplitude modulations from the EAM for different voltage biases. It is desirable for a to be maintained at a substantially constant value over desired temperature range of operation. The desired value of α is chosen based on the fiber link length. These desired α values are usually close to zero and are negative in many cases. For example, an effective α of −0.6 may be desirable for data transmission over an 80 km SMF fiber to maintain a dispersion penalty of less than 2 dB.
The α crossing point (Vcr) (the voltage at which the small signal α curve crosses through zero) can be used as a reference to maintain a constant dispersion penalty in the system. The solid lines in graph 1000 represent linear fits to the measured α crossing points of two exemplary T-Networks LIM™101 transmitter packages. As shown in this figure, the linearity of the relationship of Vcr to the operating temperature is high within at least a range of 70° C. Also, although the values of Vcr may vary significantly at a given temperature between EAM's, as shown in graph 1000, the slope of the Vcr versus temperature relationship remains virtually constant for EAM's formed from the same material system.
Desirably, the EAM may be designed so that the desired working bias voltage may be substantially equal to Vcr near the lower end of the desired temperature range, but as the temperature increases the desired working voltage may become significantly less than Vcr. The dashed lines in graph 1000 represent the desired working bias voltages for the EAM's in these packages through the desired operating temperature range of 0° C. to 70° C. These desired working bias functions may be desirably determined by the α crossing point of the lowest temperature in the temperature range (e.g., 0° C. in
The laser, SOA and EAM of an exemplary laser integrated module transmitter configuration may each be formed with a quantum well structure. It is noted that such quantum well structures may be sensitive to temperature and, therefore, sub-components designed to operate at higher temperatures in uncooled applications may not operate well at low temperatures outside of the designed range. This issue may be particularly important for EAM's. Generally during high power, long reach, operation low temperature performance of the sub-components is not an issue, but situations may exist, due to environmental or other circumstances, in which these issues may arise. One exemplary solution is to provide for temperature control through heating with either a small thermoelectric cooler (TEC) or a resistive heater mounted to the underside (or the middle of) the platform under the EAM. Reduced power budgets for both transmitters and transponders are desirable, particularly in small form factor packages. Therefore, a resistive heater may be desirable due to its efficiency of energy conversion compared to a TEC.
Therefore an uncooled, long reach transmitter or a transponder, configured in an exemplary laser integrated module configuration, may be designed to have high reliability, as well as allowing control of the dispersion penalty and output power flatness of the device. Further, such an exemplary configuration may be packaged in a small form factor package due to reduce heat dissipation requirements.
Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.
|Cited Patent||Filing date||Publication date||Applicant||Title|
|US5991060 *||Jul 24, 1997||Nov 23, 1999||Lucent Technologies Inc.||Automatic power control for a wavelength selective EML module|
|US6618176 *||Feb 26, 2001||Sep 9, 2003||Ciena Corporation||Remodulating channel selectors for WDM optical communication systems|
|US6862136 *||Jan 30, 2003||Mar 1, 2005||Cyoptics Ltd.||Hybrid optical transmitter with electroabsorption modulator and semiconductor optical amplifier|
|Citing Patent||Filing date||Publication date||Applicant||Title|
|US7187700 *||Dec 8, 2003||Mar 6, 2007||Samsung Electronics Co., Ltd.||Method for maintaining wavelength-locking of Fabry-Perot laser regardless of change of external temperature and WDM light source using the method|
|US7502567 *||Jun 15, 2004||Mar 10, 2009||Avago Technologies Fiber Ip (Singapore) Pte. Ltd.||Electroabsorption-modulated Fabry-Perot laser and methods of making the same|
|US7734189||Nov 30, 2006||Jun 8, 2010||Avago Technologies Fiber Ip (Singapore) Pte. Ltd.||Parallel channel optical communication using modulator array and shared laser|
|US7747174 *||Sep 8, 2004||Jun 29, 2010||Avago Technologies Fiber Ip (Singapore) Pte. Ltd.||Multi-channel fabry-perot laser transmitters and methods of generating multiple modulated optical signals|
|US7778552||Mar 2, 2006||Aug 17, 2010||Finisar Corporation||Directly modulated laser with integrated optical filter|
|US8285155 *||Dec 4, 2009||Oct 9, 2012||Sumitomo Electric Industries, Ltd.||Optical receiver for the WDM system and method to control the same|
|US8290376 *||Mar 16, 2011||Oct 16, 2012||Sumitomo Electric Industries, Ltd.||Optical receiver for the WDM system and the method for controlling the same|
|US9059801 *||Mar 14, 2013||Jun 16, 2015||Emcore Corporation||Optical modulator|
|US9083467 *||Nov 6, 2012||Jul 14, 2015||Fujitsu Limited||Optical transmission apparatus and optical transmission method|
|US20040208208 *||Dec 8, 2003||Oct 21, 2004||Dong-Jae Shin||Method for maintaining wavelength-locking of fabry perot laser regardless of change of external temperature and WDM light source using the method|
|US20050276615 *||Jun 15, 2004||Dec 15, 2005||Ranganath Tirumala R||Electroabsorption-modulated fabry-perot laser and methods of making the same|
|US20100142958 *||Dec 4, 2009||Jun 10, 2010||Sumitomo Electric Industries, Ltd.||Optical receiver for the wdm system and method to control the same|
|US20110243576 *||Oct 6, 2011||Sumitomo Electric Industries, Ltd.||Optical receiver for the wdm system and the method for controlling the same|
|US20130114635 *||Nov 9, 2011||May 9, 2013||Dmitri Vladislavovich Kuksenkov||Heating Elements For Multi-Wavelength DBR Laser|
|US20130156426 *||Aug 28, 2012||Jun 20, 2013||Electronics And Telecommunications Research Institute||Low power optical network terminal and method of operating low power optical network terminal|
|US20140198377 *||Dec 9, 2013||Jul 17, 2014||Omron Corporation||Laser oscillator|
|US20140270788 *||Nov 21, 2013||Sep 18, 2014||Emcore Corporation||Method of fabricating and operating an optical modulator|
|EP2854241A3 *||Sep 25, 2014||Aug 5, 2015||JDS Uniphase Corporation||Mopa laser source with wavelength control|
|WO2007103669A2 *||Feb 27, 2007||Sep 13, 2007||Hongyu Deng||Directly modulated laser with integrated optical filter|
|WO2014176865A1 *||Sep 26, 2013||Nov 6, 2014||Huawei Technologies Co., Ltd.||Optical module and preparation method therefor|
|U.S. Classification||372/50.1, 257/458|
|International Classification||H01L31/075, H01S5/12, H01S5/026, H01S5/00, H01S5/40|
|Cooperative Classification||H01S5/4006, H01S5/12, H01S5/0265, H01S5/0085, H01S5/0064, H01S5/005|
|European Classification||H01S5/026F, H01S5/40A|
|Oct 12, 2004||AS||Assignment|
Owner name: T-NETWORKS, INC., PENNSYLVANIA
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|Aug 21, 2007||AS||Assignment|
Owner name: COMERICA BANK, CALIFORNIA
Free format text: SECURITY AGREEMENT;ASSIGNOR:CYOPTICS, AQUISITION CORP. F/K/A T-NETWORKS, INC.;REEL/FRAME:019725/0366
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|May 30, 2008||AS||Assignment|
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